Gravity sewers sized for self-cleansing velocities.
Gravity sewers sized by Manning for self-cleansing velocity ≥0.6–0.9 m/s and maximum 3 m/s; manholes at changes of direction and grade; separate or combined systems. Pumping stations lift flow over high points; ventilation and corrosion (H₂S) are concerns.
A 300 mm sewer at 1:200 flows about 80 L/s when full.
Sewers must carry wastewater and stormwater by gravity, so their gradients and velocities must be set to prevent blockage and sedimentation.
Gravity sewers sized for self-cleansing velocities.
Sewers must carry wastewater and stormwater by gravity, so their gradients and velocities must be set to prevent blockage and sedimentation.
A velocity high enough to keep solids moving and prevent deposits building up in the pipe.
Gravity moves the flow, so the gradient must be steep enough to maintain velocity but not so steep that flow is too fast.
Typical applications include sizing a gravity sewer that serves a new housing estate, checking the self-cleansing velocity of a pipe carrying foul water and designing manholes and access points for maintenance on a sewer.
Study "Wastewater treatment" first (topic 82 of 112 in Civil & Structural). It covers the groundwork this topic builds on.
Minimum gradients. Peak dry weather flow. Infiltration and inflow.
A 300 mm sewer at 1:200 flows about 80 L/s when full.
The next topic is "Solid waste management", which builds directly on this one.
It is classed as Advanced in Civil & Structural (topic 83 of 112). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Civil & Structural.
Want to practise? Open the interactive course for quick checks, quizzes, simulations and progress tracking.